I recently purchased a pair of B&W P5 speakers that were made in 1997. After making sure all the old ferrofluid was removed entirely, it was time to add the magic. While adding the ferrofluid, I purposely put 60ul into on tweeter and 100ul into the other. One thing for sure, it made a huge difference in voice clarity, no more muffled sounds coming from the tweeters.

There is a slight difference between the tweeters because of the different amounts of ferrofluid used. The tweeter with 60ul is a little brighter than the 100ul, but can’t decide which one I like better. Once I decide which one to go with, I’ll even the two tweeters so they will match in sound.

Has anyone used less than the required amount of ferrofluid in a tweeter, and would there be any harm in doing so?

As far as hearing goes....Tweeters only make a ssssssssss sound, they don't handle freq's that shape mid range vocals, they do have to move very fast and handle a lot of power so tweeters are the most rugged driver in a speaker, I would put the fluid back in that is required by the manufacturer so they don't overheat during long loud periods of listening.

I also recommend that you use the proper amount of ferrofluid. Tweeters are designed with or without it for a reason. Ferrofluid has consequences of good and bad, that's why its important to use as designed. Ferrofluid does help absorb heat, so it does help with power handling. Ferrofluid also adds resistance to coil movement, so it changes QMS. Too high or too low and you will be dealing with a peak or dip. So you can see here why one would be brighter. Overall, If you don't overpower your tweeter, It should live, but my experience is that manufacturers use ferrofluid for a reason. Focal uses it when necessary. Seas doesn't use it (at least that I've seen) If your careful, you should be fine.

Pretty much all dome tweeters use ferrofluid. In addition to above points by timlub, it also helps align the voice coil. Small voice coils in a tweeter are susceptible to rocking motion.

Ferrofluid is a cheap solution to building a tweeter that will perform well for about 5 to 10 years. Usually performance declines audibly after 5 years and sometimes as little as 2 years. Replacing the ferrofluid is a good idea on 10 year old tweeters.

ATC now make all their transducers in house and their soft dome tweeter is one of very few that do not have ferrofluid. In order to overcome alignment or rocking issues they use a double spider to stabilize the voice coil and to overcome heat build up the manufacturing tolerances are very tight - costly to make.

My Hyperion HPS-938 speakers use ferrofluid for 6.5" midrange driver suspension (no spiderweb) and most likely in 1" short horn dome tweeter. They are at least 5 years old but I cannot hear any sound deterioration (I think). Hyperion Sound doesn’t exist anymore and I have no way of knowing what type or amount of ferrofluid is needed. Is replacement of ferrofluid a standard service done by the music store, speaker technician etc? What are the symptoms of dry ferrofluid?